1. Field of the Invention
The present invention relates to a radio communication system, a network side device, a small cell base station, and a transmission power control method with which uplink transmission power of a radio terminal connected to a small cell base station is controlled.
2. Description of the Related Art
Recently, a small cell base station has attracted an attention. The small cell base station is a small-sized base station which forms a small cell of a communication area with a radius of several to several tens of meters and is capable of being installed indoor. Installing small cell base stations makes it possible to disperse the traffic of a large cell base station forming a large cell of a communication area with a radius of several hundreds of meters and to cover a dead area inside the large cell.
The installation places of large cell base stations are decided by a carrier in consideration of inter-cell interference. On the other hand, a user can install a small cell base station in any place. For this reason, them is a case where a small cell base station is installed within a large cell which is formed by a large cell base station.
Under the above-described circumstance, when a radio terminal (hereinafter referred to as “small cell terminal” as needed) which is connected to the small cell base station performs communications using the same frequency band as that of a radio terminal which is connected to the large cell base station, the large cell base station receives interference from the small cell terminal. As a result, a communication rate of the radio terminal which is connected to the large cell base station is decreased.
The following uplink transmission power control method of suppressing the interference from the small cell terminal to the large cell base station has been proposed (Non-Patent Literature 1). In the method disclosed in Non-Patent Literature 1, an upper-limit value of an amount of interference from the small cell terminal to the large cell terminal is set in a small cell base station in advance. The small cell base station determines a maximum value of transmission power of the small cell terminal according to the upper-limit value and a propagation loss from the small cell terminal to the large cell base station and controls the transmission power of the small cell terminal so that the transmission power should not exceed the determined maximum value.    Non-Patent Literature 1: 3GPP TR25.967, “Home Node B Radio Frequency (RF) Requirements (FDD)”, Section 7.3, “Control of HNB uplink interference”, March 2009.